1. Field of Invention
The invention relates to a hydraulic control device for an automatic transmission.
2. Description of Related Art
Conventionally, an automatic transmission used for a vehicle, such as an automobile, comprises a transmission having a planetary gear unit and the like. The transmission comprises a plurality of friction engagement elements, such as clutches and brakes. By engaging or disengaging predetermined combinations of the frictional engagement elements, rotations are selectively outputted from gear elements, such as a sun gear, a ring gear and a carrier of the planetary gear unit, and a plurality of transmission gear ratios are realized.
The friction engagement elements are engaged or disengaged by actuating the hydraulic servo of a hydraulic circuit. The hydraulic servo is provided with a hydraulic servo drum having an outer peripheral wall and an inner peripheral wall, an annular piston slidably fitted into the hydraulic servo drum and the like. The annular piston is moved by supplying or discharging oil to/from an apply oil chamber formed between the hydraulic servo drum and the annular piston, whereby the friction engagement elements disposed to face the annular piston are engaged or disengaged.
Also, the hydraulic circuit is provided with various types of solenoid valves, switching valves and the like. If the solenoid of a predetermined solenoid valve is energized, a predetermined switching valve is switched and hydraulic pressure is thereby supplied to the hydraulic servo connected to the switching valve.
In the automatic transmission structured as described above, if a certain gear ratio is to be attained, one or more predetermined friction engagement elements are engaged. At this time, if a failure (abnormality) occurs to a solenoid valve and a frictional engagement element which should not be engaged is engaged, then interlock occurs to the transmission.
To avoid this, there is provided a hydraulic control device for preventing interlock from occurring to the transmission such that if hydraulic pressure is supplied to the hydraulic servo of the frictional engagement element which should not be engaged, then the hydraulic pressure allows a predetermined switching valve to be switched, supply of hydraulic pressure to the hydraulic servo of a predetermined frictional engagement element is stopped (see U.S. Pat. No. 4,903,551).
In the above-stated conventional automatic transmission, however, if a friction engagement element, which does not need to be engaged, is engaged, i.e., failure (onfail) occurs while hydraulic pressure is being supplied to the solenoid valve, it is possible to prevent interlock from occurring to the transmission. However, if a friction engagement element, which needs to be engaged, is not engaged, i.e., failure (off-fail) occurs while no hydraulic pressure is supplied to the solenoid valve, then the automatic transmission is shifted to neutral and sometimes cannot transmit power, with the result that the vehicle cannot travel.
Furthermore, in the conventional hydraulic control device for an automatic transmission, the switching valve is provided between a solenoid valve and a hydraulic servo. Due to this, oil leaks between the spool of the switching valve and the valve body thereof and further a valve provided between the solenoid valve and the hydraulic servo generates line resistance. If so, a predetermined hydraulic pressure cannot be supplied to the hydraulic servo. Due to this, if transient control, i.e., direct control of the engagement/disengagement of friction engagement elements by the actuation of solenoid valves is conducted, the accuracy of supply/discharge of hydraulic pressure to the hydraulic servo cannot be enhanced, thereby generating transmission shock.
To avoid this disadvantage, it may be considered to control the energization of a solenoid according to the oil leakage amount. The oil leakage amount, however, varies with the change of the valve body with the passage of time. Owing to this, as time passes, the accuracy of supply/discharge of hydraulic pressure to the hydraulic servo deteriorates.